Why don’t boats capsize?

Sean and Robot have been finding out why boats capsize and what can be done to prevent this happening.

Ships are built so that they can’t easily flip over – or capsize.

Whether a boat capsizes or not has a lot to do with something called its centre of gravity. We think of gravity as a force that pulls things downward (toward Earth’s centre), but it doesn’t always work like that. Sometimes gravity can make things turn and topple over, especially if they are high up and unbalanced.

Throw a ball in the air and gravity pulls it straight back down. Not everything moves like this when gravity acts on it. Most objects are not nice, neat shapes like balls. That means gravity acts on them in more complex ways.

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Even so, all objects behave as though their mass (the stuff they’re made from) is concentrated at a point called their centre of gravity.

A simple object like a ball has its centre of gravity in a very obvious place – right at its centre. But in a more complex object, like your body, the centre of gravity is slightly higher than your waist because there’s more weight in the top half of your body than in the bottom half. It’s like when you’re walking along on a narrow bar, you’ll know that you have to stretch your arms to help stop falling off – your centre of gravity is changing all the time as you move and moving your arms keeps you from being pulled off the bar.

If you’ve ever climbed onto a rowboat tied to a jetty, you’ll have a sense of how very unstable small boats can be when you start to load them. When you stand in a boat, you effectively become a part of it, so you alter its centre of gravity, instantly making it much higher up.

Also, since you enter a boat from the side, the minute you step on board, you shift the centre of gravity over towards the place you’re standing. Now the centre of gravity is no longer above the centre of the boat, and that makes the whole thing rotate towards you.

A boat can swing from side to side pretty freely and because it’s light, it takes only a small force to make it move in the water. All these things combined make a small boat relatively easy to capsize. By sitting down in a small boat, you lower the centre of gravity and make it more stable. Generally the lower the centre of gravity the more stable boats are.

Big boats often have heavy engines underneath the water level which gives them a low centre of gravity and makes them more stable. They also have a big keel – that’s a vertical board running underwater from back to front along the central “spine”. The keel helps stop boats rolling (moving from side to side) or capsizing, because it means more force is required to push the boat sideways through the water or rotate it.

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